Due to digitization of printing image data, in recent years, printing with a CPT system has been widely adopted in the printing industry. As a result, there has been arisen a demand for a printing plate material for the CTP system, which enables a low coast and easy handling printing while exhibiting printability comparable to that of a Pre-sensitized (PS) plate.
Specifically, there has been arisen a demand for a versatile processless printing plate having a direct imaging (hereafter referred to as DI) function in which development using a specific developer is not required and being available for a DI printer, while providing usability comparative to that of a PS plate.
A thermal processless printing plate material is imagewise exposed employing an infrared laser, with an emission wavelength in the near-infrared to infrared regions, to form an image. The thermal processless printing plate material employing this method is divided into two types; an abrasion type printing plate material and an on-press development type printing plate material with a heat fusible image formation layer.
Examples of the abrasion type printing plate material include those disclosed in for example, Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773.
These references disclose a printing plate material containing a support provided thereon a hydrophilic layer and an oleophilic layer, either of which is the outermost layer. When a printing plate material having a hydrophilic layer as the outermost layer is imagewise exposed, the hydrophilic layer is removed by abrasion to reveal an oleophilic layer, whereby an image is formed. This printing plate material tends to exhibit the problem that the used exposure device is contaminated by the ablated residue, and a special suction device is required to remove the scattered residue. Therefore, this printing plate material shows relatively lower versatility to the exposure device.
Alternatively, there has been achieved a development of a printing plate material which is capable of forming an image without abrasion, and does not require development treatment employing a special developer or wiping-off treatment. For example, there disclosed is a printing plate material for CTP in Japanese Patent Nos. 2938397 and 2938398, in which a thermosensitive image formation layer contains thermoplastic particles and a water-soluble binder which enables on-press developing by using a dampening solution or a printing ink.
However, when a grained aluminum plate is used as a hydrophilic substrate in the above mentioned on-press developing CTP system, the following problems tend to occur: (i) failure in on-press developing due to the complicated roughness of the grained aluminum plate or (ii) degradation of printing sensitivity or failure in image formation (degradation of printing durability) due to the high conductivity of the grained aluminum plate. Accordingly, it has been relatively difficult to satisfy the following three requirements: (i) on-press developing property; (ii) high printing sensitivity; and (iii) high printing durability.
Also proposed is a printing plate material having a hydrophilic layer on a support which further having thereon a water soluble layer containing a light-to-heat conversion material. Since the surface asperity of the hydrophilic layer of this printing plate material is easily controlled by selecting the size or amount of the contained particles, this printing plate material has the advantage in that a favorable surface asperity for on-press developing is easily obtained (for example, see Patent Document 1).
However, even in this printing plate material, problems of stain in non-image areas or insufficient printing durability may be found, under a certain printing condition.
Further proposed is a highly durable hydrophilic layer containing 91% by weight or more, preferably 95% by weight or more of a material which does not contain carbon (for example, see Patent Document 2). This hydrophilic layer exhibits several favorable printing properties, for example: (i) no stain in non-image area is observed even when a dampening solution containing no IPA (isopropanol); and (ii) a high anti-abrasion property and high printing durability are obtained, since it contains only a small amount of water soluble resins (the main component of the layer being metal oxide).
However, in recent years, higher printing durability is demanded in a thermal processless CTP system, and the printing durability of the above printing plate materials has become relatively insufficient.
(Patent Document 1)                JP-A No. 2001-105759        
(Patent Document 2)                JP-A No. 2002-370465        